Cellular communications have gained much popularity since 1990s. Traditionally, cellular networks are connected to public switched telephone network (PSTN) and are dedicated to voice communications. With advanced packet switching technologies, any raw signals can be formed in packets which can flow from the sender to the destination via the cellular networks and non-cellular networks. On the other hand, the manufacturing cost of cell phones, or called mobile phones, has decreased significantly, so mobile phones become affordable. It is believed that the mobile phones have penetrated more than 85% of the global population. Furthermore, more functionalities are added to mobile phones, leading the boundaries between mobile phones and personal computing devices to disappear. Many mobiles phones have now become smartphones or personal mobile computers. The smartphones allow subscribers not only to talk but also to enjoy the use of the Internet.
Due to a large volume of subscribers using smartphones, the demand of cellular transmission increases exponentially. However, the bandwidths of cellular networks are limited. A typical solution to the problem of bandwidth deficiency is to install more cellular base stations. Nevertheless, in the greater metropolitan areas, e.g., New York City, Chicago, Los Angeles, London, and Tokyo, there are sparse or no spaces to install more cellular base stations. Even though installing more base stations is feasible, users located at the “marginal-to-inoperative regions,” such as the coverage edges of base stations, hilly terrain, concrete walls, or tall buildings, still face weak or blocked signals. As a sequel, a new way to increase the cellular coverage is necessary.